JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test">
    <div class="test">
        123
    </div>
</div>

CSS

body,html
{
    height: 100%;
    margin: 0;
}

#test
{
    height: 100%;
    background-color: #000;
    position: relative;
}

.test
{
    height: 2em;
    background-color: red;
    width: 2em;
    position: absolute;
    margin: auto;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
}